The present invention is generally related to signal-conditioning methods and systems. The present invention is also related to magnetic sensor devices, such as Hall sensors and magnetoresistive devices. The present invention is also related to techniques for minimizing errors due to signal amplitude variations in magnetic sensors.
Magnetic sensing devices for detecting the presence of a ferromagnetic object in the vicinity of the sensing device are utilized widely in a variety of fields, including automotive applications. Such sensing devices typically utilize a magnetic field and employ sensing components that are capable of detecting variations in the strength of a magnetic field. Magnetic field strength generally can be defined as the magnetomotive force developed by a permanent magnet per the distance in the magnetization direction. As an example, an increase in the strength of a magnetic field, corresponding to a drop in the reluctance of a magnetic circuit, can occur as an object made from a high magnetic permeability material, such as iron, is moved toward the magnet.
Magnetic permeability is generally defined as the ease with which the magnetic lines of force, designated as magnetic flux, can pass through a substance magnetized with a given magnetizing force. Magnetic permeability can be quantitatively expressed as the ratio between the magnetic flux density (i.e., the number or lines of magnetic flux per unit area which are perpendicular to the direction of the flux) produced and the magnetic field strength, or magnetizing force. Because the output signal of a magnetic field sensing device is generally dependent upon the strength of the magnetic field, the output signal can be effective in detecting the distance between the sensing device and an object within the magnetic circuit. The range within which the object can be detected is limited by the flux density, as measured in Gauss or Teslas.
Where it is desired to determine the speed or rotational position of a rotating object, such as a disk mounted on a shaft, the object is typically provided with surface features that project toward the sensing device, such as teeth. The proximity of a tooth to the sensing device will increase the strength of the magnetic field. Accordingly, by monitoring the output of the sensing device, the rotational speed of the disk can be determined by correlating the peaks in the sensor""s output with the known number of teeth on the circumference of the disk. Likewise, when the teeth are irregularly spaced in a predetermined pattern, the rotational position of the body can be determined by correlating the peak intervals with the known intervals between the teeth on the disk.
One prominent form of such a sensing device is a Hall effect sensor. A Hall effect sensor relies upon a transverse current flow that occurs in the presence of a magnetic field. The Hall effect sensor is primarily driven by a direct current voltage source tied to electrodes at both ends of the Hall effect sensor, creating a longitudinal current flow through the sensor""s body. In the presence of a magnetic field, a transverse current is induced in the sensor, which can be detected by a second pair of electrodes transverse to the first pair. The second pair of electrodes can be connected to a voltmeter to determine the potential created across the surface of the sensor. Transverse current flow increases according to a corresponding increase in the magnetic field""s strength.
The Hall effect sensor can be mounted within and perpendicular to a magnetic circuit, which can include a permanent magnet and an exciter. The exciter can be configured as a high magnetic permeability element having projecting surface features, which increases the strength of the magnet""s magnetic field as the distance between the surface of the exciter and the permanent magnet is reduced. Typically, the exciter can be configured in the form of a series of spaced teeth separated by slots, such as the teeth on a gear. The exciter generally moves relative to the stationary Hall effect sensor element and, in doing so, changes the reluctance of the magnetic circuit so as to cause the magnetic flux through the Hall effect element to vary in a manner corresponding to the position of the teeth. With the change in magnet flux there occurs the corresponding change in magnet field strength, which increases the transverse current of the Hall effect sensor.
With the increasing sophistication of products, magnetic field sensing devices have also become common in products that rely on electronics in their operation, such as automobile control systems. Common examples of automotive applications are the detection of ignition timing from the engine crankshaft and/or camshaft and the detection of wheel speed for anti-lock braking systems and four-wheel steering systems. For detecting wheel speed, the exciter is typically an exciter wheel mounted inboard from the vehicle""s wheel, the exciter wheel being mechanically connected to the wheel so as to rotate with the wheel.
The exciter wheel can be provided with a number of teeth, which typically extend axially from the perimeter of the exciter wheel to an inboard-mounted magnetic field sensor. As noted before, the exciter wheel is generally formed of a high magnetic permeability material, such as iron. As each tooth rotates toward the sensor device, the strength of the magnetic field increases as a result of a decrease in the reluctance of the magnetic circuit. Subsequently, the magnetic circuit reluctance increases and the strength of the magnetic field decreases as the tooth moves away from the sensing device. In the situation where a Hall effect device is utilized, there should be a corresponding peak in the device""s potential across the transverse electrodes as each tooth passes near the device.
One type of magnetic sensing device utilized in automotive applications, in particular, is a magnetoresistor. In general, a magnetoresistor has higher sensitivity than a Hall element, which potentially can improve sensor performance. A magnetoresistor is a device whose resistance varies with the strength of the magnetic field applied to the device (magnetoresistance). Generally, the magnetoresistor is a slab of electrically conductive material, such as a metal or a semiconductor.
There are three different physical effects, which can cause magnetoresistance to occur. The first type of magnetoresistance is generally referred to as Anisotropic Magnetoresistance (AMR). This effect occurs in thin ferromagnetic films (on the order of several hundred Angstroms thick). The AMR effect results from deflection of magnetization of the ferromagnetic layer by an applied field, which lowers the resistance. The magnetoresistance effect is approximately 2.5% of the base resistance for permalloy (i.e., a specific alloy of approximately 78% nickel and 22% iron), which is favored because it generally is known to not possess any magneto restrictive properties. The AMR effect generally occurs in response to the in-plane component of the applied magnetic field.
The second type of magnetoresistance is referred to generally as Giant Magnetoresistance (GMR). These materials can be generally arranged in a sandwich configuration of several very thin (e.g., 15 to 25 Angstroms) alternating layers of ferromagnetic material and highly conductive material. The ferrormagnetic layers have alternating magnetization, which is rotated into alignment by an applied field to lower the resistance. The GMR effect is from 5% to 35% of the base resistance, resulting in a substantially larger signal than AMR. GMR typically responds to the in-plane component of the applied magnetic field.
The third type of magnetoresistor can be generally formed as a thin elongated body of a high carrier mobility semiconductor material, such as indium antimonide (InSb) having contacts at its ends. Such a configuration responds to the perpendicular component of the magnetic field and, because current through the slab is deflected by an applied magnetic field and flows diagonally across the slab, the resistance increases, which generally occurs as the result of the geometric magnetoresistance effect. All of these magnetoresistors can be mounted within and perpendicular to a magnetic circuit, which can include a permanent magnet and an exciter. The AMR and GMR materials simply have to be mounted further from the magnet so as to be excited by the horizontal component of the magnetic field. The exciter moves relative to the stationary magnetoresistor element and, in doing so, changes the reluctance of the magnetic circuit so as to cause the magnetic flux through the magnetoresistor element to vary in a manner corresponding to the position of the teeth of the exciter. With the change in magnet flux there occurs the corresponding change in magnet field strength, which increases the resistance of the magnetoresistor. Other types of magnetic sensors that can be utilized in such applications include AMR and GMR magnetic sensors.
Magnetic sensors such as, for example, Hall sensors or magnetoresistive devices, exhibit offset shifts due to component mismatch, calibration, temperature and aging. Additionally, electronics utilized to amplify the sensing element output before converting it to digital form also exhibit these offset shifts. One example of a magnetoresistive sensing element signal conditioning circuit is described and illustrated in U.S. Pat. No. 5,455,510, which is assigned to Honeywell Inc. The magnetoresistive sensing elements are arranged in a bridge configuration and are DC-coupled to a comparator circuit, which has switching points that vary as a function of temperature to compensate for scale factor change with temperature. The present inventor has thus concluded, based on the foregoing, that a need exists for a method and apparatus that can minimize errors due to signal amplitude variations of air gap, speed, temperature and aging in magnetic sensors such as, for example Hall sensors or magnetoresistive sensors.
AC-coupling techniques have been utilized in electronic circuits and sensors to minimize the effects of the aforementioned offset shifts. FIG. 2, for example, illustrated herein, depicts a prior art sensor signal conditioning circuit with a conventional AC-coupling circuit composed of C (capacitance) and R (resistance). The basic AC-coupling or high-pass circuit has appeared in many electronic circuit textbooks over the years. (See xe2x80x9cAnalysis and Design of Feedback Control Systemsxe2x80x9d, G. J. Thaler and R. G. Brown, McGraw-Hill, 1960, p.231, for example.) Such prior art sensor conditioning circuits have two primary disadvantages. The first disadvantage experienced by sensor signal conditioning circuits, such as the circuit illustrated in FIG. 2, is associated with noise. Noise from a differential amplifier, for example, may be coupled directly into a comparator, thus producing undesirable noise-related problems. The second disadvantage experienced by sensor signal conditioning circuits, such as the circuit depicted in FIG. 2, is related to integrated circuit implementations thereof. If a circuit such as the one illustrated in FIG. 2 is implemented as an integrated circuit, two pins are deemed necessary to make connection to the capacitor (i.e., capacitor C2), making it more susceptible to electromagnetic interference (EMI).
To appreciate the importance of AC-coupling, consider the following example: Assume a sensor output of 3 mv. Assume that the amplifier and comparator both have an input voltage offset of 1 mv and that the amplifier has a gain of 10. For DC coupling, the signal-to-offset ratio is approximately 30:11. For AC-coupling, the signal-to-offset ratio is approximately 30:1, an order-of-magnitude improvement, since the amplifier offset is removed by the AC-coupling. The offset of the comparator is the only remaining error and may be minimized as shown by amplifying the signal before applying it to the comparator. The present invention thus discloses a method and apparatus for accomplishing AC-coupling, which offers several important advantages over prior sensor-conditioning circuits.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
It is, therefore, one aspect of the present invention to provide an improved signal-conditioning circuit.
It is, therefore, another aspect of the present invention to provide an AC-coupled sensor signal conditioning circuit.
It is yet another aspect of the present invention to provide a method and apparatus for minimizing errors due to signal amplitude variations in air gap, speed, and temperature associated with magnetic sensors.
The above and other aspects can be achieved as is now described. A method and apparatus for minimizing errors in a sensor device due to signal amplitude variation are disclosed herein.
A signal output from the sensor device is amplified and thereafter AC-coupled to a comparator such that the amplification and AC-coupling of the signal minimize offset shift-related errors associated with the sensor device. The signal can be coupled to eliminate offset shifts due to mismatches, calibration, aging and/or temperature associated with the sensor device. The sensor device comprises, for example, a magnetic sensor, such as a Hall sensor or a magnetoresistive-based sensor. The signal output from the sensor device can be amplified utilizing one or more amplifiers associated with an AC-coupled sensor signal conditioning circuit.